This invention relates to a method and apparatus for transmitting digital data by a frequency division multiplex using a large number of closely-spaced carriers.
Orthogonal frequency division multiplex (OFDM) signals have been proposed for various applications, of which the first was digital audio broadcasting (DAB), also known as digital sound broadcasting (DSB); see Leonard J. Cimini, Jr., "Analysis and Simulation of a Digital Mobile Channel Using Orthogonal Frequency Division Multiplexing", IEEE Transactions on Communications, Vol. COM-33, No. 7, July 1985, pages 665 to 675. An OFDM signal consists of a large number of carriers, closely-spaced in frequency, and each modulated with data. Typically there will be several hundred or even more carriers.
The proposed DAB system involves:
(a) a Fast Fourier Transform (FFT) for both modulation and demodulation processes, to overcome frequency selectivity, using many simultaneous narrowband carriers, PA0 (b) interleaving both in frequency and in time, to realize the large coding gain in all fixed and mobile receiving conditions, and PA0 (c) coding, comprising for example convolutional coding in conjunction with a Viterbi maximum-likelihood decoding algorithm to provide a large coding gain, and possibly an outer block code concatenated with the convolution inner code. PA0 (i) Pommier and Ratliff, "New Prospects for High Quality Digital Sound Broadcasting to Mobile Portable and Fixed Radio Receivers", TBC 88, IEE Conference Publication No. 293, pages 349 to 353. PA0 (ii) Le Floch, et al., "Digital Sound Broadcasting to Mobile Receivers", IEEE Transactions on Consumer Electronics, Vol. 35, No. 3, August 1989, pages 493 to 503, which includes a block diagram of a proposed receiver. PA0 (iii) Plenge, "DAB- A New Sound Broadcasting System. . . ", EBU Review--Technical, No. 246, April 1991, pages 87 to 112, which outlines the construction of both transmitter and receiver. PA0 (iv) Casas, et al., "OFDM for Data Communications Over Mobile Radio FM Channels--Part I . . . " IEEE Transactions on Communications, Vol. 39, No. 5, May 1991, pages 783 to 793. PA0 (v) Various papers presented at the First International Symposium on Digital Audio Broadcasting, Montreux 8-9 June 1992; eg. Le Floch, "Channel Coding and Modulation for DAB", Proceedings pages 99-109 PA0 (vi) U.S. Pat. No. 4,881,241. PA0 (vii) European Patent Application EP-A-0 441 730.
Such a signal maybe termed a Coded Orthogonal Frequency Division Multiplex (COFDM).
For further background information concerning the COFDM proposal, reference may for example be made to:
In the proposed DAB system using COFDM, quadrature phase shift keying (QPSK) modulation, also termed 4-PSK modulation, is used to modulate the carriers prior to transformation by a fast Fourier transform circuit. In principle higher order PSK modulation systems could be used.
In an OFDM signal relatively high voltage peaks can occur for short periods of time when the various carriers are in phase. The ratio between the possible peak power and the mean power increases as the number of carriers increases. A problem then occurs when the signal is transmitted through a practical device (such as an amplifier), as the device must have a linear transfer characteristic with a large amount of "headroom" to prevent non-linear effects from occurring. In a transmitter network the amount of headroom required may significantly affect the cost of the transmitters. This headroom may be reduced if the instantaneous peak-to-mean power ratio of the signal can be reduced.
One method of reducing the peak-to-mean power ratio would be to use some of the data capacity of the system, so that the phase of a number of carriers can be chosen in each symbol to prevent large variations in the overall signal envelope from occurring. For DAB this is not possible because no spare data capacity exists for this purpose.
Another possibility would be to introduce a 45.degree. phase shift on half the carriers; for example, on alternate carriers. This would ensure that the carriers could not all add up in phase at once. In practice, however, we have found that little improvement is obtained this way.
It might be thought that the desired reduction in peak power could be achieved simply by limiting the signal. However, this process introduces intermodulation products. In an OFDM system, the intermodulation products fall exactly on existing carriers, and so effectively change the amplitudes and phases of these carriers.
European Patent Application EP-A-0 499 560 (and corresponding Canadian Patent Application CA-A-2 059 455) describes a receiver designed for coherent demodulation of an OFDM signal. At the receiver the received OFDM signal in the time domain is transformed by an FFT into the frequency domain. The reference data is extracted, and from this the impulse response of the transmission channel is constructed by an inverse FFT. The various components of the resultant are then weighted to discard or reduce the effect of low level components, and the frequency response then estimated by a further FFT. This estimate of frequency response is then used to coherently demodulate the received data.
In U.S. Pat. No. 5,197,061 it has been proposed to apply two different levels of protection to different bits in a COFDM system, enabling a choice to be made as to which coding system is to be applied, thus optimizing the efficiency of the transmission.
It is well known, at a transmitter which encodes a signal, also to decode the encoded transmitted signal. This is an essential part of a delta modulation system which quantises an input and compares the quantised or encoded output with the next input to generate a differential signal for transmission. In United Kingdom Patent Specification GB-A-2 188 509 it is proposed to code a video signal in two different ways, and to decode both the resultants at the transmitter. Whichever coding system introduces least impairments is then employed for the actual transmission.